Design and Evaluation of Elastic Exoskeletons for Human Running

Abstract

Humans bounce along the ground when they hop and run, providing spring-like function with their muscles and tendons. Compliant elastic mechanisms could assist this motion by contributing additional elastic storage and return. This in turn would decrease the demands on the human leg, making it easier to hop or run. I developed an elastic knee brace and an elastic lower limb exoskeleton that add parallel stiffness to the human knee joint and entire leg, respectively. The objective of this dissertation was to determine how humans are affected by the parallel elasticity when they hop and run. In the elastic knee orthosis study, ten subjects hopped on one leg with and without stiffness added in parallel to the knee. The mean brace stiffness was 5.6 N-m/◦, effectively 31.5% of total knee stiffness when hopping in this condition. When subjects hopped at fixed (2.2 Hz) and preferred frequencies, knee extensor muscle activation levels and biological knee stiffness decreased (P < .05). This indicated that elastic knee exoskeletons could be effective at reducing the metabolic cost of locomotion in bouncing gaits. However, this study also identified critical shortcomings to a joint-based approach for exoskeletons that assist running. The elastic whole limb exoskeleton was used to explore effects of adding parallel leg elasticity with a non-joint-based system. Six subjects ran with and without the exoskeleton at 2.3 m/s. While running in the exoskeleton there was a significant increase in metabolic cost as well as hip flexor and extensor muscle activation levels during the swing phase (P < .0001). The exoskeleton was designed to provide 30-50% of leg stiffness in two conditions. While running, the exoskeleton provided only 18.4% and 19.2% of leg stiffness, and only 7.0% and 7.2% of the peak vertical force transmitted to the ground. This discrepancy was due to motion of the exoskeleton waist harness on subjects and controller functionality. This dissertation provides clear suggestions for design of future exoskeletons that could assist with human running. It is expected that future devices that build on the successes of these prototypes will benefit healthy individuals and those with decreased muscle function.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/75905/1/mscherry_1.pd